Traditionally, spring cores for mattresses have consisted of a plurality of spaced parallel rows of helical coil springs mounted between border wires; coil springs adjacent the border wires being attached thereto via helical connecting wires, sheet metal clips or other connections. The upper and lower end turns of adjacent springs are generally connected to each other by helical lacing wires; the helical lacing wires being transverse or perpendicular to the columns of coil springs. Coil springs in each row are disposed in rectilinear relation to each other so as to form spaced parallel rows of springs within the border wires.
The upper and lower end turns of unknotted coil springs often are made with straight portions or legs which abut one another when coils are placed next to each other. For example, in U.S. Pat. No. 4,781,360, the end turns have deviations or offsets straightening the curved portion of the end turn so as to enable adjacent end turns to be laced together. Alternatively, the coil end turns may be rectangular with two opposite straight legs, as in German Patent No. 3,321,991. Adjacent coil springs are connected to each other at their end turns, both upper and lower, by helical lacing wire. One leg of a U-turn of an end turn of a coil spring is set beside the opposite leg of the U-shaped end turn of the adjacent coil spring. The side-by-side legs are laced together with helical lacing wire.
However, when assembled, coil springs of such a spring core may move within the lacing wire, causing misalignment or nonparallel alignment of coils in adjacent rows of coils. This misalignment causes the coil springs to line up improperly. The lines connecting the central axes of the coil springs no longer form a 90.degree. angle as they should. This misalignment thus changes a rectangular or square spring core into a rhombus. Such an odd shape must be then corrected at an additional cost.
In order to avoid this misalignment problem, spring cores were developed having individual coil springs with U-shaped end turns having one leg of a greater length than its opposing leg, as in U.S. Pat. No. 4,817,924. Once again, adjacent coil springs were connected with helical lacing wire at their end turns. However, due to the difference in leg lengths of each U-turn, the lacing wire wrapped one more time around the longer leg of the U-shaped end turn coil and one less turn around the immediately adjacent shorter leg of the adjacent end turn. The different leg lengths bound together with helical lacing wire corrected the misalignment or coil offset problem.
In U.S. Pat. No. 4,817,924, the longer leg of each U-shaped end turn is disclosed as being spaced further from the central portion of the helical spring or the axis of the spring than the distance between the short leg and the central portion or axis of the helical spring. The purpose of such spacing is to eliminate interference and noise when a load is placed on the spring core causing compression of the helical springs.
When springs are made in accordance with the disclosure of U.S. Pat. No. 4,817,924, problems may exist during manufacture. For example, because both legs of the U-shaped end turns are relatively straight, the coils may move or be skewed within the assembly machine dies prior to lacing the helical springs together. Movement or improper seating, such as results from skewing of the helical springs within the dies causes jambs and similar problems when lacing the end turns of the coils together.
It has therefore been an objective of this invention to eliminate any skewing or misalignment problems with end turns of unknotted coils within the assembly machine dies of a lacing and assembly machine.
It has been another objective of this invention to enable end turns which are to be assembled together to be more consistently gripped and positioned next to one another for lacing during the assembly of coils.
It has been another objective of this invention to provide a spring core in which the top and bottom U-shaped end turns of adjacent coil springs are bound together more tightly within or inside helical lacing wires.
It has been another objective of this invention to provide trouble-free manufacturing and assembly of unknotted coil springs by forming a small radius on the free end of each end turn of the unknotted coils, thereby reducing jambs within the assembly machine and, ultimately, cost of the resulting assembled product.